1. Field of the Invention
This invention relates to a system for determining the location of a mobile unit in a mobile, and in particular cellular, radio system.
2. Related Art
A cellular mobile radio system comprises a number of cells, each having a base station supporting a plurality of communication channels on any of which a user's call is handled until the caller passes out of range. At this point the responsibility for maintaining the call is handed over to one of a number of surrounding cells. The capacity of a cell is limited by the number of channels available. In less densely populated, e.g. rural, areas the size of the cell, which is determined to a large extent by the call concentration, is relatively large. On the other hand, in an area having a high density of mobile users, e.g. the business district of a large city, the call concentration is much greater and the cell size is relatively much smaller.
In any cellular radio system the three phases of handover are (a) deciding which cell is to receive the mobile station, (b) deciding at what point transfer should take place and (c) switching the mobile user from one base station to the other.
In high density traffic situations it has been proposed to overlay a conventional cell (a macrocell) with a network of smaller cells (microcells) created by lower power transmitters. Typically, microcells have only 200 or 500 meters range and often simply constitute a busy street, or part of a street, in which the density of calls is expected to be high.
Microcells typically consist of a group of base stations located along a busy road at 200-500 meter spacing. In the region between such a microcell and a macrocell, i.e. at the end of the road or at a turning onto another road, or between two microcells, there is a requirement for a handover.
There are at least two methods by which handover initiation is currently achieved. These are discussed in an article entitled "A handoff control process for microcellular systems" by T. Kanai and Y. Furuya, Proceedings of 38th IEEE Vehicular Technology Conference, 1988, pp. 170-175.
The first is by signal strength measurements. The base station monitors the received signal level. If the signal falls below a given threshold (or below the level of the target base station--as in the case of using relative signal strength measurement) the base station informs a central mobile switching center (MSC) that a handover to an adjacent cell is imminent.
The MSC then commands each of the surrounding base stations to measure the signal level they receive from the same mobile unit. Alternatively, the mobile unit reports continually the received signal level to the base station. The results of this are then polled and the MSC nominates which base station is to be allocated the call. The mobile unit is then commanded, via the old base station, to change to the new channel set up on the new base station.
The other method of determining when a handover should occur is by relative distance measurement based on signal delay. For example, in one known system there is a time division multiple access (TDMA) control channel. The base station transmits data to a mobile unit in timeslot 0 and receives data from the mobile unit in response some time later in, say, timeslot 3. The soliciting data sent by the base station will be received at the mobile unit after a small delay, dt. The mobile unit will then transmit to the base station in timeslot 3, which again will experience a small delay due to propagation time. The base station will expect to receive data exactly 3 slots later than it transmitted the soliciting data to the mobile unit. However, due to the delay incurred it will be received 2.times.dt later than expected. By measuring accurately the difference between expected and actual receipt of the solicited response, the distance of the mobile unit from the base station can be derived as it will be proportional to half the total delay.
In practice, the measurement of distance in a conventional cellular TDMA system between a mobile unit and its serving base station is hampered by inaccuracies due, mainly, to the lack of direct signal path. A received signal arriving at a mobile unit has usually undergone multiple reflections which will have lengthened the delay. This limits the usefulness of delay measurement in conventional cellular radio systems.
In a mixed cellular system comprising both macrocells and microcells it is desirable that microcells absorb as much traffic as possible. When a mobile unit served by a macrocell enters the coverage area of a microcell it may well be that the signal level for the macrocell base station remains sufficiently high for acceptable communications. However, it is more efficient for the system to off-load calls handled by the macrocell to a microcell whenever appropriate. It may, therefore, be difficult to decide when a handover procedure should be initiated based solely on the received signal level criteria. Furthermore, there may be situations in which it would be better to maintain the macrocell communication link with the mobile unit without initiating a handover. For example, it is not necessary to perform a handover when a mobile unit travels across a microcell for only a short period.